Composite wood products, such as board, may be formed by consolidating a loose mat of lignocellulosic materials under heat and pressure, until the materials adhere together to form a solid wood-like product. The lignocellulosic materials may take the form of wood materials, such as, particles, chips, fibers and/or the like and it will be understood that these terms are used interchangeably herein. Although it is possible to bind lignocellulosic materials under suitable heat and consolidation conditions without additional treatments, typically, the materials forming the mat are treated with a binder, such as a resin, before heat and pressure are applied, to enhance adherence of the materials and improve the resulting properties of the finished product.
Consolidation of the mat is generally conducted in a press. A conventional press for consolidating a binder treated wood composite mat to a particular molded shape, such as, for example, a board, includes two opposing press platens spaced to define a mold cavity. Typically, at least one platen is heated through conduction, such as through the use of electric heating coils or by passing a heated fluid or gas medium, such as steam, through conduits located in the platen body.
Upon contact with the mat, heat is transferred from the platen to the mat by conduction. The press platens used in a conventional press, i.e., conventional press platens, generally have a surface for contacting the mat which is free of openings or ports. Such openings in the contact surface of a platen would cause imperfections in the surface of a finished product. Thus, conventional platens are suitable for pressing boards having a "finished" surface, e.g., a surface which does not require further mechanical working or machining in post-press operations such as sanding or planing to arrive at a market ready surface. Because post-press finishing operations such as sanding or planing are not necessary for boards produced in a conventional press, the conventional press platen may be adapted to provide a smooth "finished" surface, or an embossed or patterned "finished" surface. Subsequent to removal from the press, the board may be sold as is, or the "finished" surface of the board may be treated with a protective and/or decorative coating, such as paint or stain, to yield an enhanced market ready product.
Presses using conventional press platens have several drawbacks. Presses using conventional press platens may be unsuitable for curing certain high temperature curing resins because heat transfer from a conventional platen to the inside of a mat may be slow, thus causing temperature differentials across the thickness of the mat that are unsuitable for proper curing. For example, materials near the surface of the mat may be exposed to excessive heat, causing resin to cure too quickly and composite materials to burn, thus negatively effecting such properties as appearance and bond strength. Conversely, the inside of the mat may be exposed to insufficient heat, such that the composite material does not sufficiently consolidate and the resin does not fully cure, thus weakening the internal board strength. For the foregoing reasons, i.e., heating differentials across the thickness of a mat during consolidation and/or curing resulting in negative effects on board properties, conventional press platens are unsuitable for curing relatively thick board products.
Also, although conventional presses have been successful in making fiberboard products using only conduction heat (hot pressing), today's manufacturing demands require faster cycle times on the press and the use of stronger high-temperature resins to produce highly detailed, higher density, and, at times, thicker fiberboard products. It is known that the disadvantages of conventional platens can be overcome by supplying, or injecting, steam directly into a mat through modified press platens provided with steam injection ports for that purpose. This is generally known as "steam pressing" or "steam injection pressing". The steam passes from the injection ports into interstitial spaces between the wood particles, chips and/or fibers forming the mat, thus carrying heat quickly and uniformly to the inside of the mat. Steam injection pressing has several advantages. Steam injection pressing speeds the curing of typically dimensioned boards using conventional resins, thus significantly shortening press cycles. Steam injection pressing also permits the use of high temperature curing resins, which are not typically suitable for use in conventional pressing, and which may be cheaper, safer and/or result in a stronger bonded product. And steam injection permits consolidation and curing of relatively thick composite boards, which either do not properly cure in a conventional press or do not cure quickly enough to provide a cost competitive product. Thus, steam injection is known to speed curing of composite product, improve product quality and shorten production time for wood composite products, particularly products having thick dimensions.
The benefits and advantages of steam injection can be significantly enhanced by conducting the injection in a sealed press, i.e., a press that isolates the press cavity from the surrounding atmosphere. This can be accomplished by sealing the perimeter of the cavity. Alternatively, the entire press can be isolated in a sealed chamber. A sealed press significantly reduces or eliminates the loss of valuable steam and facilitates the injection of steam into the mat at elevated temperatures and pressures.
Steam injection pressing is generally considered unsuitable for producing a "finished" surface on board products because, as noted above, ports in a press platen typically cause imperfections in the surface of the molded product. Surface imperfections must be machined or mechanically removed, by, for example, sanding or planing, in post-pressing manufacturing steps, thus adding to the cost and complexity of manufacture. In addition to steam injection ports, steam injection platens may have channels in the mat contact surface to direct the flow of injected steam over the mat surface to various parts of the mat.
In a process referred to herein as "single-sided" steam injection, a mat is pressed between a single steam injection platen (a platen having steam injection ports) and a conventional platen free of steam injection ports. Steam injected through the single steam injection platen speeds curing of the mat and shortens press cycles. The conventional platen in a single sided steam injection process avoids undesirable imperfections in one surface of the molded product that would typically result from the steam injection ports of a steam injection platen.
Effective steam pressing of composite board products can only occur if the steam passes freely into every part of the mat to uniformly heat the mat to the saturation temperature of the steam and cure the binder. For example, in a fiberboard mat having a specific gravity of less than 0.7, intersticial spaces, i.e., spaces between the fibers, are relatively large and uniform steam penetration of the mat is relatively easily achieved. However, in a fiberboard mat having a specific gravity greater than 0.7, the relatively smaller intersticial spaces act like long, narrow channels. Subsequent to pressing the mat to this higher density, the intersticial spaces may contain air. Steam does not mix freely with the air in such narrow channels, but instead pushes the air through the channel until the air is released from an open end of the channel, or trapped in a blocked channel. Fibers and binder adjacent to the trapped air are not contacted by the steam, and consequently do not cure properly. The improper curing of some portions of the mat yields a composite product with flaws in, for example, strength and appearance.
The process of single-sided steam injection of composite board having a high specific gravity is particularly susceptable to the problem of trapped intersticial air. This is because intersticial spaces or channels are more likely to be blocked by the conventional platen which has no ports to permit escape of trapped intersticial air. Furthermore, in single-sided steam pressing, due to the use of a ported platen and a platen without ports, a cumulative effect occurs. Air in intersticial spaces in the uncured mat is pushed by a steam front moving from the steam injection platen towards the conventional platen free of steam injection ports. Simultaneously, a second steam front from moisture converted to steam by the conduction heat of the conventional platen may push trapped air towards the steam injection platen. Thus, the air is trapped in the core of the mat, generally closer to the conventional platen than the steam injection platen if the steam is injected under pressure. The air is unable to vent or escape through the conventional platen, which has no ports, and is trapped between the injection steam front and the conventional platen, or between the injection steam front and the second steam front. If the process is conducted in a sealed press, the problem is compounded by the inability of the trapped air to escape through the edges of the mat. The trapped air blocks steam from contacting and fully curing the binder. Furthermore, the trapped air may cause "blowouts" and other imperfections in the finished product. The resulting board has inferior physical properties.
U.S. Pat. No. 4,162,877, issued to D. W. Nyberg discloses a steam-injection pressing system which includes two opposing press platens defining a molding cavity into which a fibrous mat is positioned and pressed to a desired shape. Only a lower platen is a steam distribution and injection platen which includes conduits supplying injection ports to provide fluid communication between the molding cavity and both an external source of steam and a venting system, separated by controlling valves. The upper platen includes no injection or venting ports or nozzles.
In operation of the system of U.S. Pat. No. 4,162,877, after a fibrous mat is positioned within the molding cavity, steam from the steam supply is introduced through the conduits and ports of the lower platen and injected into the pressed fibrous mat located within the molding cavity. After a selected period of time, the control valves are operated to close off the supply of steam and thereafter to open the molding cavity to the venting system. The venting system uses the conduits and injection ports of the distribution and injection platen to draw steam and moisture from the molding cavity.
Since the opposing (upper) platen of U.S. Pat. No. 4,162,877 is "clean", it may be used as an embossing platen to impress detail into the pressed fibrous mat, but only if the mat has a density less than 0.7. At any higher mat density, according to the patent, a mesh must be used to help prevent air from becoming trapped adjacent to the upper platen. Unfortunately, for many embossing-press applications, the density of the fibrous mat is greater than 0.7 and any use of a wire mesh, as taught by U.S. Pat. No. 4,162,877 would preclude the use of an embossing surface plate in the opposing platen.
It is known that trapped air can be removed or vented from a mat by "flushing" steam through the mat. Steam injected into a mat is passed through the thickness of the mat and exhausted from the mat such that it pushes or carries trapped air out of the mat. Air can be "flushed", for example, through the edges of the mat. However, flushing steam out through the edges of the mat is inefficient in the production of some dimensional lumber due to the relatively small edge area relative to a large surface area of a mat in contact with press platens. Flushing steam through the edges is also not suitable in sealed press applications or in high density mats in which flow is restricted. Alternatively, steam can be injected into the mat from one injection press platen and exhausted through an opposite press platen provided with ports to establish a "cross-flow" of steam across the thickness of the mat. U.S. Pat. No. 4,684,489 for a process for making composite wood panel calls for compression without steam injection to a first compression position, subsequent steam pressing with intermittent "flushing" of steam from one injection platen to an opposite injection platen, final compression with steam injection from both platens and a vacuum step. Although this existing "cross flow" press design allows steam to heat all areas of the mat evenly and effectively, it precludes the use of an embossing platen wherein one surface of the cavity remains "clean", free of any injection nozzles, meshes, grooves, or openings, i.e., so that high detail may be embossed on the surface of the compressed mat. This process is therefore not suitable for the production of board having at least one "finished" surface.
A journal publication to Ernest W. Hsu titled A Practical Steam Pressing Technology for Wood Composites, Proceedings of the Washington State University International Particleboard/Composite Materials Symposium, Pullman, Washington, Apr. 10, 1991 (hereinafter "Hsu 1991"), generally discloses that steam injection is suitable for making thick board products. Hsu also teaches that "if injection is delayed, the mat for a high density panel may become too compressed for effective steam penetration, particularly if steam pressure is low."
In U.S. Pat. No. 4,393,019 to Geimer it is disclosed that by taking advantage of the natural porosity of the mat, press time can be reduced by transferring heat to the mat connectively. According to Geimer, a well known method of using convective heat transfer is the "steam shock" or "steam jet" method wherein a mat laden with surface moisture is contacted with hot platens which vaporize the moisture. The steam created moves quickly toward the center of the mat, thereby raising core temperature. Geimer goes on to discuss the introduction of steam directly into the mat (applied steam) as a separate method of heating and curing mats. Geimer also teaches that steam introduced into the mat penetrates between particles, flakes or fibers and actually creates or opens permanent paths by which heat can transfer by convection to the center of the board (col. 4, lines 1-6). Geimer '019 does not teach combining the "steam shock/jet" step with a venting step and a steam injection step, and does not recognize the problem of trapped air.
In general, the prior art recognizes but does not fully address the problem of uncured zones caused by trapped air in a compressed mat in a process suitable for producing thick boards having at least one finished surface. Thus, there is a need for a single-sided steam injection process that can produce a thick board with suitable strength and consistency and with at least one finished surface.